RNA ligase joins covalently RNA strands in the absence of template direction. The enzyme has been identified in bacterial, mouse, and human cells. To determine the function of the enzyme in vivo we plan to isolate and characterize mutants of bacteriophage T4 which fail to induce the enzyme. A pilot study has indicated that the enzyme can also join single-stranded DNA molecules to RNA and DNA acceptors. We will investigate the properties of this reaction in detail to develop a new method for synthesis of DNA and DNA-RNA copolymers. The method will be applied to the synthesis of a defined DNA oligonucleotide and the joining of non-complementary molecules of macro-molecular DNA.